1. Field of the Invention
This invention relates to a head-up display apparatus, and particularly to a head-up display apparatus by which the observation of information is effected through a beam coupler having a hologram.
2. Related Background Art
A head-up display apparatus for observing thereby the display information from the display device and the landscape ahead in spatially superposed relationship within the same field of view by the use of an optically transparent light beam coupling element such as a half-mirror provided with a reflecting surface comprising a multilayer film or a hologram element comprising a volume type phase diffraction grating is well known.
Several applications of such head-up display apparatus to the cockpits of aircrafts and various vehicles have heretofore been proposed.
FIG. 1 of the accompanying drawings is a cross-sectional view showing the head-up display apparatus shown in U.S. Pat. No. 4,218,111. In FIG. 1, the reference numeral 61 designates a display element for producing a display pattern, the reference numeral 62 denotes the light emitting surface (the displaying surface) of the display element 61, the reference numeral 63 designates a relay lens installed forwardly of the light emitting surface 62, the reference numeral 64 denotes the real image of the light emitting surface 62 formed by the relay lens 63, the reference numeral 65 designates a turn-back prism inclinedly installed forwardly of the relay lens 63, the reference numeral 66 denotes a light beam combining element comprising a reflection type hologram, and the reference numeral 67 designates the observer's pupil.
The light emitted from the light emitting surface 62 has a narrow band width chiefly of a particular wavelength .lambda. and the relay lens 63 forms the real image 64 of the light emitting surface 62 by this light, and the light from the real image 64 is reflected by the turn-back prism 65 and enters the reflection type hologram 66.
The reflection type hologram 66 is designed to reflect chiefly a light of narrow band width in the vicinity of the wavelength .lambda. and transmit therethrough chiefly lights of wavelengths other than the narrow band width wavelength range, and forms the virtual image of the light emitting surface 62 on the pupil 67.
Accordingly, the observer can observe the information displayed on the light emitting surface 62 in superposed relationship with the landscape ahead, through the hologram 66, while seeing the landscape.
However, in the head-up display apparatus shown in FIG. 1, the display element 61 uses a CRT device having the interior of its tube kept in a vacuum which produces a light beam of narrow band width chiefly of the particular wavelength .lambda. and therefore, if an attempt is made to contrive an enlarged area of the light emitting surface 62, the volume and weight of the entire display element will be remarkly increased and after all, this has led to the problem that the entire head-up display apparatus becomes bulky. Also, the luminance of the fluorescent substance of the light emitting surface 62 is limited, and when the landscape is bright, visibility is reduced. Also, the CRT device used in the apparatus of this kind to produce a light beam having the abovedescribed narrow band width is an expensive device and precludes a reduction in the cost of the head-up display apparatus.